Mass transfer devices are used in various medical applications where it is necessary to effect a molecular transfer from one fluid to another. Examples are dialyzers and blood oxygenators.
Blood oxygenator systems are widely used in open heart surgeries and for providing emergency cardiopulmonary assistance. In both cases, the oxygenator takes over, either partially or completely, the normal gas exchange function of the patient's lung. In oxygenators which employ a microporous membrane, blood is taken from the patient and is circulated extracorporeally through the oxygenator on one side of the membrane. Carbon dioxide is transferred from the blood across the microporous membrane into the passing stream of oxygenating gas. At the same time, oxygen is transferred from the oxygenating gas across the membrane into the blood. The circulating blood, having thereby been reduced in carbon dioxide content and enriched in oxygen, is returned to the patient. Blood is circulated, oxygenated and returned to the patient in the aforementioned manner until the patient's own cardiopulmonary system is once more able to carry out its normal circulatory and gas exchange functions.
Several types of blood oxygenators have been or are generally available. One type is a bubble oxygenator wherein the oxygenating gas is introduced into the blood directly in the form of bubbles. In a second type of oxygenator, called a film-type oxygenator, a thin blood film is made and gas exchange takes place on the surface of the exposed blood film. A third type of blood oxygenator is called a membrane oxygenator. In the membrane oxygenator, the blood is separated from direct contact with the oxygenating gas by a membrane. This membrane must be microporous or semipermeable, that is, the membrane must be capable of permitting carbon dioxide and oxygen to permeate through it while at the same time preventing the fluid itself from passing therethrough.
There are two main types of membrane blood oxygenators currently available. One type, called the flat plate membrane oxygenator, employs one or more thin, flat sheets of microporous membrane. In its most basic form the flat plate oxygenator has a single sheet of microporous membrane sealed into a housing so as to provide in the housing a blood compartment for the flow of blood and a separate gas compartment for the flow of an oxygenating gas. Each of the compartments is fitted with an inlet and an outlet. Blood flows into and out of the blood compartment and the oxygenating gas flows into and out of the gas compartment. Oxygen passes from the oxygenating gas across the membrane into the blood flowing through the blood compartment. Carbon dioxide passes from the entering blood across the membrane to be entrained in the oxygenating gas. The exiting blood, now reduced in carbon dioxide and enriched in oxygen, is returned to the patient.
The other main type of membrane oxygenator is the hollow fiber oxygenator which may be constructed in several different ways. One construction type is illustrated generally in U.S. Pat. No. 4,239,729 to Hasegawa et al. A hollow fiber oxygenator employs a large number (typically, thousands) of microporous or semipermeable hollow fibers disposed within a housing. These hollow fibers are sealed in the end walls of the housing which are then fitted with skirting end caps. One end cap is fitted with an inlet and the other end cap is fitted with an outlet. The peripheral wall of the housing has an inlet located interiorly of one of the end walls and an outlet located interiorly of the other end wall. In the Hasegawa et al. oxygenator, the hollow fibers are aligned in the housing so that their longitudinal axes are generally parallel to the longitudinal axis of the housing. Blood enters through the inlet of one end cap, passes through the lumens of the hollow fibers, and exits through the outlet of the other end cap. The oxygenating gas enters the device through the inlet in the peripheral wall near one end of the device, passes over the outer surfaces of the hollow fibers, and exits the device through the outlet in the peripheral wall near the other end of the device. It will be understood that carbon dioxide diffuses from the blood flowing inside the hollow fibers through the fiber walls into the stream of oxygenating gas. At the same time, oxygen from the oxygenating gas flowing over the outer surfaces of the hollow fibers diffuses through the walls of the hollow fibers into the lumens thereof to oxygenate the blood flowing therethrough.
Another type of hollow fiber oxygenator utilizes a spirally wound hollow fiber bundle. This type hollow fiber oxygenator is illustrated in U.S. Pat. No. 4,975,247 to Badolato et al. The Badolato et al. oxygenator includes a hollow fiber bundle having first and second ends located within an oxygenator chamber. The oxygenator chamber includes a hollow core around which the hollow fibers are spirally wound and an outer casing adjacent the fibers. A gas entry port is coupled to the interior of the fibers adjacent the first end of the bundle and a gas outlet is coupled to the interior of the fibers at the second end of the bundle. A blood inlet to the oxygenator chamber, exterior of the fibers, is provided adjacent the second end of the bundle, and a blood outlet from the oxygenator chamber is provided adjacent the first end of the bundle. In contrast to the fluid flow pattern of the Hasegawa et al. oxygenator, in the Badolato et al. oxygenator, carbon dioxide diffuses from the blood flowing over the outer surfaces of the hollow fibers through the fiber walls into the stream of oxygenating gas flowing through the lumens of the hollow fibers. At the same time, oxygen from the oxygenating gas flowing inside the hollow fibers diffuses through the walls of the hollow fibers to oxygenate the blood flowing over the outer surfaces of the hollow fibers.
Another example of a spirally wound oxygenator is illustrated in U.S. Pat. No. 4,690,758 to Leonard et al.
In the past, fiber bundles for spirally wound mass transfer devices including oxygenators have typically been made by winding hollow fibers circumferentially around a generally cylindrical core. The winding is repeated until the desired bundle thickness is obtained. This results in a generally cylindrical fiber bundle. The interiors of the fibers are then accessed in some manner so that an inlet and outlet may be added in order to provide a fluid flow path through the lumens of the fibers. In the Badolato et al. oxygenator this was done by embedding the fiber bundle at its top and bottom ends in a solidified potting composition and then cutting a portion of the top and bottom ends of the bundle to expose the lumens of the fibers. The same general procedure is utilized in U.S. Pat. No. 4,690,758 to Leonard et al.
In U.S. Pat. No. 4,715,953 to Leonard a hollow fiber separation device is disclosed in which the lumens of the fibers are accessed in a different manner. In this device hollow fibers are circumferentially wrapped about a core which is typically cylindrical and hollow to form a fiber bundle. The fiber bundle is then impregnated with a potting compound and centrifuged until the potting compound is cured. A longitudinal space is cut through the circumferentially wrapped bundle at the potted band, exposing open ends of the hollow fibers at opposed sides of the space. The resulting shape of the bundle of hollow fibers wrapped about the core typically assumes the configuration of a split ring.
Regardless of the manner in which the fiber bundles are constructed or the method used to access the lumens of the fibers, the method of construction has typically been complicated, time consuming and somewhat wasteful of materials. For example, in those oxygenators where the lumens of the fibers are accessed at the top and bottom ends of the cylindrical bundle two potting steps and two cutting steps are required resulting in additional construction time and in waste of materials. Although in Leonard U.S. Pat. No. 4,715,953, only one potting and cutting step is required, a significant amount of fiber is cut away as waste. Additionally, the resulting structure wherein the gas inlet and outlet are combined into one manifold is undesirable in that communication leaks between the gas meet and outlet can develop compromising the efficiency and safety of the oxygenator.
In view of the disadvantages in the construction and operation of prior art spirally wound hollow fiber bundles currently used in mass transfer devices, a mass transfer device utilizing a hollow fiber bundle which can be constructed efficiently from both a time and material standpoint would be desirable.